1. Field of the Invention
This invention relates generally to aircraft
Traffic Alert and Collision Avoidance Systems and, more particularly, to the determination of target aircraft tracks utilizing the Air Traffic Control Radar Beacon System.
2. Description of the Related Art
The increased demands placed on the aircraft flight deck as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring of aircraft traffic in a vicinity of an aircraft that includes automatic identification of potential threats to the monitoring aircraft. As a result, aircraft can have transponders associated therewith that, in response to appropriate electromagnetic interrogation signals, provide responding electromagnetic signals. The responding electromagnetic signals include information with respect to the range, altitude, and heading of the interrogated aircraft. Certain Traffic Alert and Collision Systems (e.g., the mode-S system) include target identification as part of the information imposed on the responding electromagnetic signals. For these mode S systems, the identification of the path or track of the responding aircraft is relatively simple, involving time dependent positions and altitudes of an identified aircraft. Similarly, extrapolations or extensions of aircraft tracks can be relatively simple, at least in concept.
In the situation in which the responding electromagnetic signals do not include aircraft identification information imposed thereon, the determination of the aircraft flight path is more complicated. The information obtained by periodic interrogation of a multiplicity of unidentified targets, with associated range, altitude and bearing (heading) information being provided as a result of the interrogation, can be subjected to well known algorithms to provide a target aircraft track, i.e., the path of the target aircraft relative to the monitoring or surveillance aircraft as represented by target aircraft reply communications in response to a interrogation signals from the monitoring aircraft. Once the track is identified, then the extension thereof can be computed to determine if the target aircraft is a threat to the monitoring aircraft.
The track determination is complicated for several reasons, the reasons generally involving spurious target images. For example, referring to FIG. 1, a monitoring aircraft 10 can transmit an interrogation (electromagnetic) signal, INTERROGATION, to target aircraft 15. The transponder in target aircraft 15 provides a response signal RESPONSE. (The delay between the transmission of the interrogation signal and the response signal provides the range information). However, the interrogation signal INTERROGATION can provide a response signal RESPONSE' that is reflected from the earth's surface 19. The RESPONSE' signal, reflected once from the earth is generally referred to as a single bounce multipath signal. Because the length of time for the travel of the RESPONSE' signal is longer than for the RESPONSE signal, the RESPONSE' signal can be interpreted as separate target aircraft at a greater range from the monitoring aircraft. Similarly, an interrogation signal INTERROGATION' signal can reflect off the surface 19 of the earth, activate the transponder of the target aircraft 15 and provide a response signal RESPONSE" signal that also reflects oil the earth's surface. Because both the INTERROGATION' signal and the RESPONSE" signal are each reflected once from the earth's surface, this signal is referred to as a double bounce multipath signal. The RESPONSE" signal will be interpreted by the monitoring aircraft as a target aircraft at a greater range than indicated by the RESPONSE signal or the RESPONSE' signal. FIG. 2 illustrates the situation where a single target aircraft is providing the monitoring aircraft with a plurality of target responses during each interrogation period. FIG. 2 also illustrates another feature of the Air Traffic Control Radar Beacon System. The target image replies (RESPONSE, RESPONSE', or RESPONSE") identified by the monitoring aircraft can be missing. In the algorithms for analyzing tracks, a preselected number of target replies can be missing (e.g., because of electromagnetic interference, apparatus limitations, etc.) and the track algorithms will continue to provide extensions (extrapolations) for the previously identified tracks even in the absence of target replies. After the preselected number of track replies are missing during the periodic interrogation by the monitoring aircraft, the track is no longer considered to be present.
Other spurious replies can be generated, the spurious replies compromising the track generating algorithm. The ATCRBS system can use a Whisper-Shout mode of interrogation in which two interrogation signals of different amplitude are used to interrogate the target aircraft transponders. Between the threshold of the transponders and the suppression (delay) of response when interrogation signals greater than a defined amplitude are received by the transponder, the Whisper-Shout mode provides assistance in separation of target replies, but the Whisper-Shout mode can provide spurious replies.
Multipath tracks that are coasted (extended) can have rats of change that are in excess of the rates of change of true target tracks because of multipath tracks do not have a linear range rate with time, but the multipath tracks are usually extended in a linear manner. Also, if the normal range jitter associated with replies causes a higher than nominal range rate prior to coasting a track, the coasted multipath track will tend to overtake in range the true target track. This effect can have deleterious consequences with respect to track extension of the ATCRBS system. One example of how the track extension can fail is illustrated in FIG. 2. Four sets of interrogation signals are generated by a monitoring aircraft a time t.sub.1, t.sub.2, t.sub.3, and t.sub.4. The interrogation signals, in this example, produce multiple target replies associated with each interrogation signal. At time t.sub.1, the target replies A(1), A(2) and A(3) are detected. At time t.sub.2, the target replies are B(1), B(2), and B(3) are detected. At time t.sub.2, three tracks have been recognized by the program, track(1) through target replies A(1) and B(1), track(2) through target replies A(2) and B(2), and track(3) through target replies A(3) and B(3). At the next interrogation time t.sub.3, target replies C(1) and C(3) are detected. Track(1) is correlated with target reply C(1), while track(3) is correlated with target reply C(3). Track(2), having no target reply that is correlated therewith, is allowed to "coast", i.e., use the predicted value rather than an actual target reply range measured value for t.sub.3 to continue, or extend the track. In this example, the slope of track(2) is greater than the slope of track(1) so that at interrelation period t.sub.4, the value of track(2) is predicted to have a lower range value than track(1). At time t.sub.4, the target replies that are identified are D(1), D(2), and D(3). In the extension procedure, each target reply is assigned to a track beginning with the smallest range value, the closest potential treat to monitoring aircraft. Following this procedure, the target reply D(1) is assigned to track(2), an assignment which is incorrect and which will require a period of time to recognize and correct.
A need has therefore been felt for a procedure of correlating target replies with tracks in a Air Traffic Control Radar Beacon System which provides less possibility of an erroneous correlation.